1. Field of the Invention
The invention relates to steam generators of the type comprising a pressure vessel containing a stack of heat exchange elongated elements and whose upper part forms a steam plenum and a feedwater header located in the vessel above the heat exchange elements, generally of toric ring shape, connected to a feed water intake pipe passing through the wall of the vessel and having nozzles for distributing the flow of feed water into the vessel. It is particularly suitable for use in nuclear power stations and more especially in power stations using a water cooled and moderated reactor.
2. Prior Art
The steam generators of PWRs frequently comprise a feedwater header, whose shape approximates that of a toric ring, fed by a substantially horizontal pipe section. During normal operation of the reactor, the free surface of water in the vessel is above the pipe and the ring and the speed of the water in the pipe is relatively high. Operation is then satisfactory. But it may be disturbed under different temporary exceptional conditions.
In particular, different transitory operating phases lead to the appearance of pressure surges causing harmful overpressures. This occurs when the water level in the steam generator (for example in the case of shut down at high temperature) drops below the level of the feed water pipe. The header is then unflooded. If this situation lasts, the header and the pipe empty and fill up with saturated steam. When power operation is resumed, under-saturated water is fed through the duct: it causes sudden condensation of the steam, so a depression then the formation of a pressure wave which travels towards the pipe in the form of a pressure surge.
Attempts have already been made to overcome this problem by preventing the header and/or the pipe from emptying. For that, the conventional distribution holes provided at the lower part of the distributor have been replaced by J-shaped discharge tubes connected to the upper part of the distributor (French Pat. No. 2 333 200 and U.S. Pat. No. 4,502,419). This arrangement solves the problem, but it is complex and difficult to use as a retrofit in existing steam generators.
Another source of cracks is thermal stratification of the feed water during low pressure operation. This phenomenon is described in the article "Loading conditions in horizontal feed water pipes of LWRs influenced by thermal shock and thermal stratification effects", by M. Miksch et al, NUCLEAR ENGINEERING AND DESIGN, 84 (1985), 179-187. This phenomenon is more acute in steam generators having a preheated water supply used for normal operation and a cold water supply used under abnormal situations and at low pressure. In the later situation, a low cold water feed supply flow is injected into the pipe which is full of water at the operating temperature of the stream generator. Since the flow speed is low, the cold feed water does not mix with hot water present in the pipe, but forms a distinct layer at the lower portion of the pipe. Stratification maintains a high temperature gradient between the lower part and the upper part of the wall of the pipe, which generates thermal stresses likely to cause cracking, including in the weld connecting the pipe to the enclosure. This phenomenon is further aggravated by the fatigue induced by the oscillations of the separating layer.
The J-shaped tubes used for attenuating the hammering phenomenon do not appreciably reduce this second phenomenon, suggesting that it is maintained by the heat exchange due to natural convection between the hot water in the vessel and the inside of the pipe. Attempts have been made to overcome this second problem. One consists in injecting feed water into the pipe through a ring weir opening into a water box where mixing occurs to a limited extent. It has also been proposed to provide, on the internal face of a bend of the pipe an annular or spiral weir of small height for increasing the turbulence of the flow. Such a device has only a low efficiency: when the flow is small, i.e. when mixing is desired, the steam of cold feed water just jumps over the obstacle.
Similar problems exist in the BWRs, where the feedwater header is located within the pressure vessel of the water, with similar consequences.